Large scale sedimentation units for the continuous gravity separation of particulate matter suspended in a liquid (generally water) find wide spread use in several industries including mining and treatment of wastewater in various industrial, environmental and chemical processing operations. Depending on whether the desired end product is the liquor or the suspended solids, such units are known either as thickeners for increasing the concentration of suspended solids such as ores, or clarifiers for separating waste solids and obtaining a purified liquid.
The art of thickening or clarifying a solids-in-water system by continuous gravity sedimentation is very well established. Such sedimentation units typically comprise a large sized circular or rectangular basin having a bottom sloped around a central outlet and receiving a low turbulence feed of the solid-liquid system to be separated. As the feed liquid stands in the basin, gravity over time concentrates suspended particles at the bottom. The concentrated solids layer is then raked by a rotating raking mechanism to a discharge cone and pumped as underflow from the unit. Concurrently, a liquid which has been clarified by the settling action is discharged as overflow from the basin and captured by a launder attached to the perimeter of the basin.
A major operational difference between a thickener and clarifier is typically seen in the solids concentration of the feed. A thickener will generally have a higher solids concentration which increases the torque requirement of the raking mechanism. A clarifier generally receives a feed having a low solids concentration and a small particle size. As a result, chemical flocculators are commonly employed.
Drawbacks to the present art of continuous gravity sedimentation are significant. The raking mechanism and other submerged equipment have relatively high maintenance costs. Abnormal resistance resulting from surges in dense-feed solids, excessive amounts of oversize solids, sloughing of solids from the sides of the basin or other obstructions to the passage of the raking members can cause such members to become stuck or buried and overload the drive. Also, the solids discharge lines which are also generally submerged can become plugged. Maintenance of submerged equipment generally requires the basin to be drained and can result in long operational down times. In addition, the raking mechanism requires high torque drive units which are very costly.
Therefore, a need exits for a continuous gravity sedimentation unit which can be operated without high torque drives and associated submerged equipment for reduced maintenance problems and costs.